Good question Todd.
Generally speaking, there's two spine terms. Tilt and angle.
When someone talks about their spine tilt, it's a lateral tilt - either towards or away from the target. This is similar to leaning left or right.
When someone talks about their spine angle, it's the forward/backwards tilting (bending over or standing up).
So, in the Two Plane Swing, your spine
angle is upright, and your spine
tilt is away from target.
In the One Plane Swing, your spine angle is more bent over, and your spine tilt is centered or square (neither towards or away from target).
The one other thing that a One Plane Swing may do is contradict a static fitting chart. I find, because I'm more bent over, that 2º flat works better for my club lie. A static fitting chart says I should be either standard or 1º up. (And the
Ping Web Fit figures I should be 2º up).